Electret motors capable of continuous rotation

ABSTRACT

An electric motor comprises a rotor including a plurality of electret portions and a stator including an array of fixed electrode pairs for driving the electret rotor in response to an applied AC voltage. A significant advantage of this motor is the fact that it does not require either wound conductive coils or a laminated metal core; consequently, it can be more easily fabricated than typical prior art motors. In a preferred embodiment, the rotor comprises a rotatably mounted circular disc including a plurality of electret portions disposed at equal angular intervals around the rotor disc. The stator can comprise a pair of fixed discs, having a plurality of opposing electrode portions.

FIPEZSUZ KR.

United States Patent Anderson et a1.

[54] ELECTRET MOTORS CAPABLE OF CONTINUOUS ROTATION Inventors: Wilmer C.Anderson, Greenwich; Robert W. Windebank, Fairfield, both of Conn.

General Time Corporation, Phoenix, Ariz.

Filed: July 30, 1970 Appl. No.: 59,513

Assignee:

US. Cl ..310/5, 310/10 Int. Cl. ..H02n 1/00 Field of Search ..310/2, 5,6, 46; 324/109 [56] References Cited UNITED STATES PATENTS 2,232,1432/1941 Schweitzer ..324/l09 X OTHER PUBLICATIONS 7/1961 Devol ..3l0/46 X[4 1 Oct. 3, 1972 QC585.68, PP. 130,131. Publication, Electrety;Myazdrikof & Manoilof; Moscow 1962 QC585,M57 pp. 80- 83.

Use of Electrets in Electrical Instruments by Andrew Gemant, Rev. ofScientific Instruments, Vol. 11, 1940, pages 65- 71.

Primary Examiner-D. X. Sliney Attorney-Pennie, Edmonds, Morton, Taylor &Adams [5 7] ABSTRACT An electric motor comprises a rotor including aplurality of electret portions and a stator including an array of fixedelectrode pairs for driving the electret rotor in response to an appliedAC voltage. A significant advantage of this motor is the fact that itdoes not require either wound conductive coils or a laminated metalcore; consequently, it can be more easily fabricated than typical priorart motors. In a preferred embodiment, the rotor comprises a rotatablymounted circular disc including a plurality of electret portionsdisposed at equal angular intervals around the rotor disc. The statorcan comprise a pair of fixed discs, having a plurality of opposingelectrode portions.

12 Claims, 14 Drawing Figures "i 's i 15--fi L :1 u 9 a Ac l xul QPATENTEDnm 3 I972 SHEET 2 [IF 4 INVENTORS ROBERT W. WINDEBANK WILMER CANDERSON ATTORNEYS ELECTRET MOTORS CAPABLE OF CONTINUOUS ROTATIONBACKGROUND OF THE INVENTION The present invention relates to an electretelectrical motor capable of continuous rotation.

Electrical motors are among the most important and best known means forconverting electrical energyeither DC or AC-into useful mechanicalenergy. These motors typically utilize the interaction between magneticfields in a fixed member (stator) and a rotatably mounted member (rotor)to drive a shaft. The magnetic fields are produced by current carryingconductive coils wound into slots in the stator and rotor. At least oneof the members, i.e., either the stator or rotor, is provided with meansfor applying electrical power to its coils. Current is applied to thecoils of the other member either directly by connection with anelectrical power source or indirectly by magnetic induction.

These motors, however, are relatively expensive and difficult tofabricate. The coils generally comprise lengthy portions of insulatedwire-typically enameled copper-wound within the recesses of thesupporting members, and the supporting members are usually built of thininsulated laminations of silicon steel (from about 0.025 to 0.014 inchthick) having uniformly distributed punched slots to receive the coilwindings. It is clear that this structure presents relatively complexfabrication problems, such as the winding of the coils in the stator androtor members, and requires numerous repetitive operations, such as thestamping out of the numerous steel plates required for a single member.

SUMMARY OF The INVENTION In accordance with the present invention, anelectric motor comprises a rotor including a plurality of electretportions and a stator including an array of fixed electrode pairs fordriving the electret rotor in response to an applied AC voltage. Asignificant advantage of this motor is the fact that it does not requireeither wound conductive coils or a laminated metal core; consequently,it can be more easily fabricated than typical prior art motors. In apreferred embodiment, the rotor comprises a rotatably mounted circulardisc including a plurality of electret portions disposed at equalangular intervals around the rotor disc. The stator can comprise a pairof fixed discs having a plurality of opposing electrode portions.

BRIEF DESCRIPTION OF THE DRAWINGS grams of the motors shown in FIGS. 1,4, and 6, respec tively;

FIGS. 38, 5B, and 7B are graphical illustrations showing the appliedvoltage as a function of the angular position of a given electret polewhen the motors of FIGS. 1, 4, and 6, respectively, are in operation;and

FIG. 8 is a breakaway view showing a motor in accordance with theinvention adapted to utilize a plurality of rotor discs to drive acommon shaft.

In reference to the drawings, FIG. 1 shows an electret motor comprisinga rotor disc 10 including one or more electret poles 11 uniformlydistributed around the periphery of the disc. The stator comprises apair of dielectric plates 12A and 12B having one or more pairs ofelectrodes, 13A and 138, on respective discs for driving the rotor inresponse to an applied AC voltage. The rotor 10 is rotatably mountedbetween stator discs 12A and 12B on a shaft 14 which can pass throughthe stator to drive a motion converting mechanism, such as gear 15,which can be used, for example, as the drive gear of a clock train. Lowfriction bearings (not shown) are positioned at each end of the shaft.Preferably, the shaft maintains the rotor axially spaced from each ofthe electrodes 13A and 13B by an equal distance, d, so that the axialforces cancel one another.

As is well known in the art, electrets are dielectric materials in whicha permanent state of electrostatic polarization hasbeen established bythe displacement of the positive charges in constituent atoms withreference to negative charges. They are typically produced by subjectinga plastic dielectric material, such as Mylar, to heat, applying a highelectric field across the heated plastic and permitting the material tocool while the field is applied. The properties of and the methods forfabricating electrets are explained in detail in the literature of theart. See, for example, US. Pat. No. 3,118,022 issued to G. M. Dessler onJan. 14, 1964. Electrets for the instant application can be of anydielectric which can be electrostatically polarized to a sufficientlyhigh voltage-typically at least 400 volts to be useful in motors. Mylar,polycarbonate, and polytetrafluorethylene are typical examples of suchmaterials.

Rotor 10 is preferably a circular disc having a plurality of electretsattached at equal angular intervals around the circumference. It canconveniently comprise any one of a wide variety of structures. FIGS. 2A,2B, 2C, and 2D show four representative examples. FIG. 2A illustrates arotor comprising a single circular electret 20 in which sectors betweenadjacent electret poles 11 have been punched out or otherwise removed.

FIG. 2B shows a rotor comprising a circular dielectric disc 21 to whichpairs of oppositely oriented electrets have been attached on oppositesides at each electret pole. The resulting composite structures behavevery much like single electrets. FIG. 20 shows a rotor comprising adielectric disc in which sectors of electret material have beensubstituted for corresponding punched out sectors of dielectric. AndFIG. 2D illustrates a rotor comprising a circular electret disc havingdielectric layers 22 and 23 covering all but the desired electret poles20. In some embodiments of the invention, it is useful to alternate thepolarity of successive electret poles. This can be readily accomplishedin the rotors of FIGS. 2B and 2C by simply turning alternate electretsupside down. While the above illustrations show only eight-pole rotors,they have been made with four, eight, and 16 poles. In general, in orderto obtain a desired speed, the rotor can have any other number of poles(even or odd) which is practical.

The stator comprises a pair of dielectric plates having a plurality ofelectrode pairs for driving the rotor in response to an applied ACvoltage. In the illustrative embodiment of FIG. 1, eight opposingelectrode pairs are used in conjunction with eight electret poles. Ingeneral, there are usually at least as many electrode pairs as electretpoles so that each pole is driven by at least one such pair. Inmultiphase motors, it is often desirable to utilize a number ofelectrode pairs which is an integral multiple of the number of electretpoles.

The stator plates may be conveniently fabricated by well known printedcircuit techniques. For example, the stators can comprise epoxy printedcircuit boards having electrodes of the same general configuration asthe electret poles uniformly distributed around the circumference of acongruent circle. The electrodes can be fabricated with appropriateelectrical interconnectrons.

Advantageously, the stator plates are bound by a member (not shown),such as an axially extending cylinder, to form a hermetic seal aroundthe rotor. This seal prevents variations in humidity from altering theoutput torque of the motor.

The operation of the motor can be more easily understood by reference toFIGS. 3A and 3B which show a schematic circuit diagram of the motor ofFIG. 1 and a plot of an applied AC voltage as function of the angularposition B of a moving rotor. As may be seen from the drawing, the ACvoltage tends to drive the rotor at such an angular velocity that Ncycles, where N is the number of electret poles, drive the rotor through360. Ideally, the AC voltage drives the rotor at such a velocity thatthe midpoint of the electret poles tends to pass the midpoints of theelectrodes as the AC voltage wave passes through one of its zero voltagepoints, and begins to grow positive. At this point in time, both theupper and lower electrodes repel the rotor, driving it in the samedirection in which it was moving. The rotor poles approach the midpointbetween adjacent electrode pairs as the wave again changes polarity andis subsequently attracted toward the next successive electrode pair. Themotor will behave in substantially the same manner if the AC voltage hasa DC component.

It will be noted that the motor can be driven in either direction. Adirection preference, however, can be easily established by displacingone of the rotor poles slightly in the direction of the desired motion.

As a specific example, an electret motor utilizing eight electret poleshaving a built-in field of 2,000 volts and disposed 1.5 centimeters fromthe rotor center is typical. If this motor is driven by a 100 volt ACdrive (e.g., at 60 c.p.s.), the output torque is on the order of 385gram-millimeters-an amount sufficient to drive a typical clock train orvarious indicating devices.

FIG. 4 shows an electret motor in accordance with the second embodimentof the invention. It differs from the motor shown in FIG. 1 chiefly inthat it utilizes successive electrode pairs of opposite polarity inconjunction with successive electret poles of opposite polarity. Theadvantage of this type of motor is that each electret pole, in additionto being repelled by the electrode pair it has just passed, it alsoattracted by the next succeeding pair. Thus, the output torqueapproaches twice the corresponding value in the previously describedmotor.

The operation of this motor can be more easily understood by referenceto FIGS. 5A and 5B which show, respectively, a schematic circuit diagramof the motor and a plot of applied AC voltage as a function of the rotorangle. It may be noted from the drawings that the applied voltage drivesthe rotor at such an angular velocity that N/2 cycles drive the rotorthrough 360. More specifically, as in the previous motor, the appliedvoltage tends to drive the rotor at such a velocity that the electretpoles tend to pass the electrode midpoints as the AC voltage wave passesthrough zero voltage points. Unlike the previous motor, however, therotor poles approach the midpoint between adjacent electrode pairs asthe wave approaches a maximum (or a minimum) value.

FIG. 6 shows a third embodiment of the invention comprising an electretmotor adapted for a two-phase AC drive voltage. This motor differs fromthe one shown in FIG. d chiefly in that it includes two electrode pairsfor each electret pole. AC voltages having a 90 phase difference fordriving successive electrode pairs are conveniently provided by an ACsupply connected to a phase shifter 60 such as the well-knownresistancecapacitance 90 phase shifter shown. In addition, successiveelectrode pairs of the same phase are of opposite polarity. Theoperation of this motor may be seen by reference to FIGS. 7A and 7Bwhich schematically illustrate the motor circuitry, including a phaseshifter, and the drive voltage as a function of the rotor angle. Thevoltages tend to drive the rotor so that the pole centers pass themidpoints of each pair of electrodes just as the applied voltagereverses polarity. As in the case of the motor of FIG. 4, the voltageapplied to successive electrode pairs of the same phase is at a maximumor a minimum when the pole is midway between them.

FIG. 8 shows yet a fourth embodiment of the invention for use inapplications requiring greater torque than can be obtained from a singlerotor. It comprises a plurality of electret rotors disposed on a commonshaft 81. The rotors are sandwiched between a plurality of statormembers 83 each of which includes one or more electrodes (not shown)aligned to form a plurality of electrode pairs for driving the rotors.The stators are conveniently attached to a housing 84 with electrodesinsulated therefrom. In this arrangement, the shaft 81 can thus bedriven by a relatively large number of rotors each contributing to thetotal output torque.

I claim:

I. An electric motor capable of continuous rotation comprising, a rotormechanically coupled to a rotatable shaft, said rotor comprising a dischaving a plurality of electret poles uniformly distributed around thecircumference in a circular pattern, and a stator including a pair ofplates positioned on each side of said rotor and being coaxially alignedtherewith, each plate having a plurality of electrodes uniformlydistributed about the circumference thereof in a circular pattern.

2. A motor according to claim 1 wherein the circular patterns of saidelectret poles and said electrodes are of substantially the same radius.

3. A motor according to claim 2 wherein the electrodes of the statorplates are opposite one another to form electrode pairs.

4. A motor according to claim 3 wherein said rotor is rotatably mountedbetween said stator plates at a position substantially equidistantbetween them.

5. A motor according to claim 4 wherein there are at least as manyelectrode pairs as electret poles.

6. A motor according to claim 5 wherein the number of electrode pairsequals the number of electret poles.

7. A motor according to claim 6 wherein successive electret poles havethe same polarity.

8. A motor according to claim 6 wherein;

successive electret poles have inverted polarities;

and

means are provided for inverting the polarity of the AC voltage appliedto successive electrode pairs.

9. A motor according to claim 1 adapted to operate in response to amultiphase AC voltage.

10. A motor according to claim 9 wherein the number of electrode pairsis an integral multiple of the number of electret poles.

1 1. A motor according to claim 10 wherein;

the number of electrode pairs is twice the number of electret poles;

successive electret poles are of inverted polarity; and

means are provided for applying to successive electrode pairs, ACsignals having a phase difference, and for inverting the polarity ofsuccessive electrodes receiving AC signals of the same phase.

12. An electric motor capable of continuous rotation comprising:

a plurality of rotors coupled to a common rotatable shaft, each of saidrotors including one or more electret poles; and a stator including aplurality of electrodes for driving said rotors in response to anapplied AC signal.

1. An electric motor capable of continuous rotation comprising, a rotormechanically coupled to a rotatable shaft, said rotor comprising a dischaving a plurality of electret poles uniformly distributed around thecircumference in a circular pattern, and a stator including a pair ofplates positioned on each side of said rotor and being coaxially alignedtherewith, each plate having a plurality of electrodes uniformlydistributed about the circumference thereof in a circular pattern.
 2. Amotor according to claim 1 wherein the circular patterns of saidelectret poles and said electrodes are of substantially the same radius.3. A motor according to claim 2 wherein the electrodes of the statorplates are opposite one another to form electrode pairs.
 4. A motoraccording to claim 3 wherein said rotor is rotatably mounted betweensaid stator plates at a position substantially equidistant between them.5. A motor according to claim 4 wherein there are at least as manyelectrode pairs as electret poles.
 6. A motor according to claim 5wherein the number of electrode pairs equals the number of electretpoles.
 7. A motor according to claim 6 wherein successive electret poleshave the same polarity.
 8. A motor according to claim 6 wherein;successive electret poles have inverted polarities; and means areprovided for inverting the polarity of the AC voltage applied tosuccessive electrode pairs.
 9. A motor according to claim 1 adaPted tooperate in response to a multiphase AC voltage.
 10. A motor according toclaim 9 wherein the number of electrode pairs is an integral multiple ofthe number of electret poles.
 11. A motor according to claim 10 wherein;the number of electrode pairs is twice the number of electret poles;successive electret poles are of inverted polarity; and means areprovided for applying to successive electrode pairs, AC signals having a90* phase difference, and for inverting the polarity of successiveelectrodes receiving AC signals of the same phase.
 12. An electric motorcapable of continuous rotation comprising: a plurality of rotors coupledto a common rotatable shaft, each of said rotors including one or moreelectret poles; and a stator including a plurality of electrodes fordriving said rotors in response to an applied AC signal.